1. Field of the Invention
The present invention relates to a technology for forming an image using electrophotography.
2. Description of the Related Art
Image density formed by an image forming apparatus typically varies affected by various influences. To cope with this problem, a density adjustment technology for stabilizing the image density is widely known, in which a patch for measuring density (hereinafter, “patch pattern”) is formed on an image carrier, image density of the patch pattern is measured to obtain a developing characteristic, and image-forming conditions, such as a developing bias or an amount of exposure, that affect the image density are adjusted according to the developing characteristic.
When the density is adjusted, there is a problem in that a downtime occurs from when the patch pattern is formed to when the image density is detected, during which a user cannot make prints. Three types of conventionally known technologies for coping with the problem are disclosed in, for example, Japanese Patent Application Laid-open No. H08-95460, Japanese Patent No. 3924375, and Japanese Patent Application Laid-open No. 2002-116586. In the technology disclosed in Japanese Patent Application Laid-open No. H08-95460, density adjustment is performed during a warm-up of a fixing unit, i.e., a period of time until the fixing unit is heated to a predetermined temperature. In the technology disclosed in Japanese Patent No. 3924375, an aging operation that can be implemented within a warm-up of a fixing unit until when the fixing unit is heated to a predetermined temperature is performed. In the technology disclosed in Japanese Patent Application Laid-open No. 2002-116586, density adjustment is performed by forming patch patterns by changing a laser power, obtaining density information about the patch patterns, and selecting a method of density adjustment that can be implemented within a warm-up of a fixing unit, i.e., a period of time until the fixing unit is heated to a predetermined temperature.
However, in the technology disclosed in Japanese Patent Application Laid-open No. H08-95460, when the warm-up time for fixing is short, a downtime occurs caused by the density adjustment. In the technology disclosed in Japanese Patent No. 3924375, the downtime caused by the density adjustment does not occur, but it is impossible to adjust a factor for density control other than an amount of charge of a developer. In the technology disclosed in Japanese Patent Application Laid-open No. 2002-116586, the downtime caused by the density adjustment does not occur, and the factor for density control other than the amount of charge of the developer can be adjusted; however, it is difficult to obtain an accurate developing characteristic within a limited time because the patch-pattern forming condition is selected from among several alternatives prepared in advance.
The reason for difficulty in obtaining the accurate developing characteristic within a predetermined time when the patch-pattern forming condition is selected from among the alternatives prepared in advance is described below. The only way to form the patch pattern within a limited time is to reduce the number of patch patterns. The developing characteristic (relation between laser power and density) may possibly be non-linear characteristics, and therefore, the patch pattern near the target density needs to be formed for obtaining an accurate developing characteristic. However, in the technology disclosed in Japanese Patent Application Laid-open No. 2002-116586, because the number of laser power values to be ready is merely considered in terms of determining the patch-pattern forming condition, it is difficult to form the patch pattern near the target density even when fixed patch-pattern forming conditions prepared in advance are used.